1. Field of the Invention
The present invention relates generally to a method and an arrangement for detecting overcurrent conditions and more specifically to a control circuit including provisions to determine the validity of a sensed-current signal before issuing a trip signal to interrupt the current.
2. Description of the Related Art
Various control circuits are known which are responsive to the current in an AC line for detecting the presence of overcurrent conditions. For example, see U.S. Pat. Nos. 4,605,982, 4,571,658, 4,567,540, 4,642,724, 4,104,691, and 4,002,950, as well as U.S. application Ser. Nos. 791,195, 791,199 and 905,891 filed in the name of J. W. Ruta. While a current-sensing transformer is utilized in these control circuits to provide an accurate representation of instantaneous current for a portion of each half-cycle of current after zero crossing, the current is not accurately represented after the current-sensing transformer saturates. Additionally, various current-sensing arrangements are disclosed in U.S. Pat. Nos. 4,434,415, 4,297,741, and 4,532,571 and in a publication entitled "Digital EHV Current Transducer" by L. B. Berkebile, 1980, IEEE, 80 SM 647-8.
The control circuit in U.S. Pat. No. 4,685,024 utilizes a digital processor for performing RMS sampling of circuit current for comparison to stored long-time and short-time current values and corresponding time delays before tripping a breaker. A magnetic-core transformer is employed for providing an analog sensed current signal to the digital processor. A core saturation signal is generated when the sensed current exceeds 10 times the rated circuit current since that is the current at which the current transformer is known to begin to saturate. When the core saturation signal is provided, the digital processor utilizes the largest possible current for the overcurrent determination rather than the actual current.
Where iron-core transformers or other magnetized-core transformers are utilized to sense the current, saturation can occur at the higher current levels which causes the sensed-current signal to be inaccurate. Of course, such inaccurate sensed-current signals can cause the generation of false trip signals; i.e., the generation of a trip signal to interrupt current when in fact the actual current is not at the level at which the tripping mode is desired. In many applications, it is impractical to provide a transformer with a large enough magnetic core to produce accurate sensed-current signals for the higher currents at which overcurrent detection takes place.
Thus, while these arrangements recognize the problem of obtaining an accurate representation of the sensed current, these arrangements neither provide a suitable instantaneous sensed-current signal nor determine the validity of a sensed-current signal before a decision is made to generate a trip signal and interrupt the current.